Lau Anna, Blitzblau Hannah, Bell Stephen P
Howard Hughes Medical Institute, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Genes Dev. 2002 Nov 15;16(22):2935-45. doi: 10.1101/gad.764102.
Transcriptional silencing in Saccharomyces cerevisiae involves the assembly of a heterochromatic domain that is heritable from generation to generation. To maintain the silenced state, the propagation of silent heterochromatin must be coordinated with the events of chromosome duplication and segregation. Here we present an in vivo analysis of the cell-cycle events required for the establishment of the silenced state at the HMR silent mating-type locus. We show that Sir protein recruitment to and spreading from the HMRE silencer is poor during S phase, but is robust during G2 and subsequent phases. Despite abundant Sir protein association in cells arrested in G2/M phase, silencing is not fully established by this stage of the cell cycle. Rather, robust silencing is not observed until telophase. Interestingly, the elimination of the cohesin subunit Scc1/Mcd1p allows the full establishment of silencing to occur in G2/M phase. Furthermore, expression of a noncleavable allele of Scc1/Mcd1p inhibits the establishment of silencing. Our findings reveal both S- and M-phase requirements for the establishment of silencing and implicate the loss of sister-chromatid cohesion as a critical event in this process.
酿酒酵母中的转录沉默涉及一个可代代遗传的异染色质结构域的组装。为维持沉默状态，沉默异染色质的传播必须与染色体复制和分离事件相协调。在此，我们对在HMR沉默交配型位点建立沉默状态所需的细胞周期事件进行了体内分析。我们发现，在S期，Sir蛋白募集到HMRE沉默子并从其扩散的情况较差，但在G2期及后续阶段则很活跃。尽管在G2/M期停滞的细胞中Sir蛋白大量结合，但在这个细胞周期阶段沉默并未完全建立。相反，直到末期才观察到稳定的沉默。有趣的是，去除黏连蛋白亚基Scc1/Mcd1p能使沉默在G2/M期完全建立。此外，Scc1/Mcd1p的不可切割等位基因的表达会抑制沉默的建立。我们的研究结果揭示了建立沉默对S期和M期的需求，并表明姐妹染色单体黏连的丧失是这一过程中的关键事件。
